The present invention is in the field of explosive and thermic compositions or charges. More specifically, the present invention relates to plastic bonded explosive compositions and methods for preparing such compositions.
Coating techniques and equipment for producing plastic bonded explosive (PBX) molding powders are long known in the art. For example, see the Encyclopedia of Explosives and Related Items (S. M. Kaye, US Army Armament Research and Development Command, 1978, vol. 8, pages 62-65). Specifically, the Kaye reference at pages 64-65 and in FIG. 1 describes a slurry method for preparing PBX granules using an aqueous slurry.
Generally, in the aqueous slurry coating process, a high explosive (HE) powder is suspended in water in a reactor vessel with sufficient agitation or stirring to form an aqueous HE powder slurry. A lacquer containing a polymeric binder dissolved in an organic solvent (e.g., ethyl acetate) is added to the aqueous HE powder slurry with continuous agitation to form a mixture. The lacquer in the mixture, which is xe2x80x9csticky,xe2x80x9d causes the particles of the HE powder to agglomerate. To complete the agglomeration of the HE powder, the solvents are removed from the mixture, leaving hardened granules of polymer coated explosivexe2x80x94or molding powder.
Water is the preferred slurry medium, and the aqueous slurry coating process is most effective when the lacquer solvent is partially miscible/soluble with water. However, other slurry mediums are used when the suspended solids to be slurried are immiscible with water, or the lacquer solvent is insoluble in water. An example of the prior case is the manufacturing process for Magnesium-Teflon-Vitron (MVT) flare composition, where the finely powdered magnesium component is incompatible with water, n-Heptane is used as the slurry medium. An example of the latter case is the coating of HE powders (e.g., Composition CH-6) where the polymeric binder (polyisobutylene) is dissolved in a lacquer solvent (n-Octane) that is substantially insoluble in water. In this latter case, the aqueous HE powder slurry based agglomeration process can result in granules that are not well rounded.
The field has been motivated to develop alternatives to the aqueous slurring processes to address these limitations. For example, Chan et al., U.S. Pat. No. 5,750,921, describe a slurring process for producing HE molding powders using a non-aqueous medium (a liquid fluorocarbon) to suspend the initial explosive powder. Although the Chan process may be useful for its intended purpose, it would be beneficial to have an alternative aqueous slurring process that can avoid using fluorocarbons and utilize a lacquer solvent that is substantially insoluble in water and still produce HE molding powders having well rounded granules.
One of the benefit of an aqueous HE powder agglomeration process to produce PBX molding powder is the avoidance of using a substantial volume of potential pollutants that an analogous, but wholly organic solvent based agglomeration process can involve. However, it can be difficult to use the aqueous HE powder slurry process to form agglomerates with some polymeric binders, because the lacquer solvent is not soluble in the water phase of the aqueous slurry. For example, in the Ethyl vinyl acetate (EVA) coating of explosive powders, certain grades of EVA (specifically EVLAX 40W(trademark)) are dissolved using n-Butyl acetate. However, n-butyl acetate is substantially insoluble in water: 0.7 percent by weight at room temperature. During the removal of the solvents from the reaction (or agglomeration) mixture in the coating process, the agglomeration of the HE powder can fail or be lost, likely because of the insolubility of n-Butyl acetate in water.
The present invention relates to lacquer solution composition (stock solutions) useful in a coating process for producing HE molding powders coated with a binder that is solublized in a water immiscible solvent. Specifically, it is a coating process for producing HE molding powders utilizing an aqueous slurry medium and a lacquer stock solution comprising a solvent that is substantially immiscible in water, to produce HE molding powders having well rounded granules. The present invention uses the discovery that an HE agglomeration process utilizing an aqueous HE powder slurry comprising Ethyl acetate, upon the addition of a lacquer containing a substantially insoluble binder solvent, n-Butyl acetate, produced well rounded molding powder granules that retained their shape as the solvents were removed in the coating process.
The improved results embodied in the reaction mixture solutions of the present invention may have been accomplished by one or a combination of unexpected benefits resulting from the inclusion of Ethyl acetate to the aqueous HE powder slurry before addition of the lacquer to the slurry. One is that the adjustment of the surface tension of the aqueous phase by the Ethyl acetate, another is the modification of the wetting characteristics of the slurry, and still another is that the Ethyl acetate acts to entrain the n-Butyl acetate, making it more soluble in the water phase. The result was to produce agglomerate granules that were hardened or strengthened sufficiently to withstand the mechanical manipulation of agitation/stirring during the agglomeration process.